A directional coupler is a measurement tool that samples a small portion of radio frequency and microwave energy traveling through a coaxial cable. The coupler can be thought of as two separate parts: 1) a circuit to sample the voltage in the coaxial cable, and 2) a current probe to sample the current in the coaxial cable. These samples provide two voltages, which add for current in one direction and subtract for current in the opposite direction, thereby giving the coupler its directivity properties.
There are three basic types of directional couplers: a 3-port uni-directional coupler, a 4-port bi-directional coupler, and a 4-port dual directional coupler. The uni-directional coupler consists of a transmission line and an auxiliary line, which is internally terminated at one end, with the other end acting as the coupling output. It is necessary to reverse the coupler physically for both forward and reverse power measurements. The bi-directional coupler is similar to the uni-directional coupler with the exception that both ends of the auxiliary line act as coupled outputs. This type of coupler can be used for simultaneously monitoring both forward and reflected power, but the directivity, which is the ratio in decibels of the power coupled out in the preferred direction to the power coupled out in the opposite direction, is dependent on the loads on the coupled outputs. The dual directional coupler employs two uni-directional couplers internally connected in tandem, to provide coupled outputs for both forward and reflected power. Unlike the bi-directional coupler, the directivity of the dual coupler is unaffected by the loads on the coupled outputs.
An example of an existing dual directional coupler is a coupler consisting of a transmission line extending between mainline input and output ports in the form of coaxial connectors. The transmission line has a center conductor surrounded by a dielectric layer, and the transmission line is enclosed in a solid housing, which serves the dual purposes of an outer conductor and an enclosure. The relative dimensions of the center conductor, dielectric and the housing are chosen to form a characteristic impedance, usually 50 ohms. The housing has two holes aligned with the center conductor, and two tubes each have open ends slidingly positioned within the holes. The opposite ends of the tubes extend out of the housing and terminate in coaxial connectors, which act as coupled outputs. A non-inductive termination resistor and pick-up link are placed across each of the tubes' open ends. The resistor is electrically connected to the tube, while a resistor lead on the opposite end of the resistor forms the pick-up link and bends up at a right angle into the tube and is connected to an amplitude equalizing circuit. The equalizing circuit comprises capacitance and a series resistance and is connected to the coupled output.
One tube of this directional coupler and its circuitry forms the forward auxiliary line, while the second tube and its circuitry forms the reverse auxiliary line. Coupling, which is the ratio in decibels of the forward power in the main transmission line to the power appearing at the coupled output, is adjusted by moving the tubes in and out of the housing to adjust the proximity of the pick-up links to the center conductor. Directivity is adjusted by rotating the tubes to properly align the pick-up links with the center conductor. These adjustments are somewhat interactive, requiring repeated fine tuning to achieve a desired result. In addition to the cumbersome adjustment procedure, this type of coupler suffers from at least four limiting factors: (1) the pick-up links are short, limiting the range of coupling factor, and suitability to frequencies below 100 MHz; (2) power handling must be sacrificed for tighter coupling; (3) the series resistive element in the coupled port dissipates power but is not heat sunk; and (4) the terminating resistor connected to the inside of the tube can not be effectively heat sunk.
A more recently developed dual directional coupler overcame many of the limiting factors of the above-described coupler. The coupler includes a larger housing containing a main transmission line, and one side of the housing is open to employ air as a dielectric and a circuit board is fixed within the housing. Pick-up links are placed parallel to the center conductor of the transmission line. The coupling factor is adjusted by changing the proximity of the links to the center conductor. Once adjusted, the pick-up links are actually permanently imbedded in the dielectric layer of the transmission line. Thereafter, each end of the links are bent at right angles, pass through the circuit board and are connected to amplitude equalizing circuits mounted partially on an opposite side of the circuit board and partially on the housing. The equalizing circuits are connected to coupled outputs.
This more recent coupler was an improvement over the prior dual directional coupler because: (1) the pick-up links are not limited in length, allowing tighter coupling, the only requirement being that the links be electrically short, or less than 1/4 wavelength, for the frequencies involved. The coupler, therefore, is useful to frequencies as low as 1 MHz. In addition, with the transmission line and pick-up links contained within a larger housing, more room is available for the equalizing circuitry, allowing the use of more complicated networks without series resistance. The result is a coupler capable of wide-band operation over frequencies extending into the medium frequency and high frequency ranges; (2) power handling does not have to be sacrificed for tighter coupling, and the coupler is able to handle four times greater power for any given set of parameters than the above described coupler. For any given power requirement the coupling can be 6 dB tighter with this arrangement; and (3) any elements dissipating heat can be and are effectively heat sunk to the housing.
Adjusting the coupling factor, however, is difficult with this more recent coupler, requiring that the pick-up links actually be moved independently of the remainder of the auxiliary lines, permanently set or imbedded in the dielectric layer and then soldered to the remainder of the auxiliary lines. In addition, once the coupling factor is initially set, it very difficult to readjust the coupling since the original dielectric layer would have to be destroyed and replaced to move the imbedded pick-up links, which would than also have to be re-soldered to the remainders of the auxiliary lines. The link arrangement is also costly and time-consuming to manufacture, especially when the coupler is required to endure severe shock or vibration.
What is desired, therefore, is a dual directional coupler having forward and reverse coupled outputs that can be precisely, simultaneously, quickly and easily set to an exact coupling factor. Preferably, the coupler will be permanently aligned, have exceptional bandwidth and power handling ability and be capable of handling large amounts of average and peak power. The coupler should allow all power dissipating elements to be mounted to a common heat conductor so that all heat is transferred via conduction to coupler's housing. In addition, the housing of the coupler should be substantially shorter than the housings of existing quarter wavelength couplers, yet allow the use of complicated equalizing circuits and not limit the length of the pick-up links. The coupler should be rugged and able to withstand shock and vibration, be inexpensive to manufacture and its design should allow the coupler to exhibit lower tolerances, and consistent performance and quality from unit to unit.